A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat.
In a proton exchange membrane (PEM) fuel cell, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton-conducting membranes such as those based on perfluorosulphonic acid materials are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, typically comprising a platinum-based electrocatalyst. An electrocatalyst is a catalyst that promotes the rate of an electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion material. The gas diffusion material is porous and electrically conducting. It allows the reactants to reach the electrocatalyst layer and conducts the electric current that is generated by the electrochemical reactions.
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion material to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane. Subsequently, gas diffusion materials are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion material adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
The electrocatalyst layers usually contain proton-conducting polymer in contact with the electrocatalyst reaction sites. This enables the efficient transport of protons from the anode reaction sites through the polymer membrane to the cathode reaction sites. Incorporation of proton-conducting polymer in the catalyst layer can improve catalyst utilisation, i.e. the proportion of the platinum-based catalyst that actually takes part in the catalytic reaction is increased. The catalyst utilisation is affected by the three-phase interface between the catalyst, the gaseous reactants, and the proton-conducting polymer. Improving the catalyst utilisation can increase the MEA performance (measured as cell voltage at a given current density) without increasing the amount of platinum-based catalyst.
One method of incorporating proton-conducting polymer into an electrocatalyst layer is a method wherein an electrocatalyst ink containing electrocatalyst, a proton-conducting polymer and a solvent is prepared, and the ink is applied to a suitable substrate such as a gas diffusion material, a membrane or a transfer film. Another method of incorporating proton-conducting polymer into an electrocatalyst layer is a method wherein a dispersion of proton-conducting polymer is applied to a pre-formed electrocatalyst layer. EP 731 520 discloses methods of preparing electrocatalyst layers using electrocatalyst inks and/or proton-conducting polymer dispersions, wherein the solvent in the ink or dispersion is predominantly aqueous. It is desirable to use aqueous dispersions and/or inks in industrial manufacturing processes because problems associated with handling and disposing of high volumes of organic solvents are overcome.